Brush assembly for dynamoelectric machines



July 2, 1940. A. G. REDMOND BRUSH ASSEIBLY FOR DYNAMOELECTRIC IACHINES Filed Oct. 22, 1937 Gum mp Patented July 2, 1940 UNITED STATES PATENT OFFICE BRUSH ASSEMBLY FOR DYNAMOELECTRIC MACHINES 3 Claims.

This invention relates to certain new and useful improvements in dynamoelectric machines and more particularly to the brush assembly therefor, the object being to provide a brush so '5 constructed that brush rattle or vibration will be practically eliminated without increasing the cost of manufacturing the same.

The improved construction of brush and holder is particularly adapted to be used in the manufacture of direct current, 6 volt fractional horse power motors as used to operate the fans of heaters for automobiles, although it can be used with the same advantage in large motors of either the alternating current or direct current type.

In the mass production of fractional horse power type of motors using a stationary brush holder with a brush movably mounted therein and held in contact with the commutator by spring pressure, it is essential that sufiicient clearance be maintained between the outer wall of the brush and the inner Wall of the holder to allow the brush free longitudinal movement therein and as the holder varies several thousandths of an inch and the brush varies several thousandths of an inch and the diameter of the commutator varies several thousandths of an inch and run excentrically, it is impossible to fit these parts together and obtain perfect alignment or accuracy as it is practically impossible to have the necessary clearance to obtain free longitudinal movement without creating sidewise vibrations when the motor is running which creates a noise known in the art as brush rattle caused by the brush vibrating in the holder as the commutator passes over the same and it has been found by extensive experimenting that this noise can be eliminated by beveling the end of the brush in contact with the commutator to cause 40 the brush to cook within the holder and wedge which prevents sidewise movement without interfering with the free longitudinal movement.

With a brush constructed with an inner end beveled in two directions and struck on the arc of a circle, a point is provided having a small area of contact between the brush and commutator so as to allow the brush to wear quickly to conform to the shape and position of the commutator whereby the slots of the commutator 50 are allowed to slide under the contacting area of the brush without setting up vibrations and without interfering with the proper operation of the motor.

It has been found by experimenting extensively 55 that brush noise is developed mainly from four sources: (1) the brush edge dropping into the slots between the commutator bars; (2) the brush vibrating and coming in contact with the walls of the brush holder; (3) creating noise due to roughness of the commutator after being subjected to uneven wear due to vibrating brushes; 5 and (4) electric noise due to poorly fitting brushes.

In order that small motors can be manufactured cheaply, it is essential that the parts of these motors be manufactured without machin- 10 ing the same and without precision in the construction and precision in fitting the parts together and it has been found that where the brush holder and the brush do not vary to any great extent that the vibration of the brush can be eliminated by simply forming the end of the brush with a beveled end to cause it to cock within the holder.

It has also been found that a brush constructed with an end beveled in two directions has many advantages over other constructions of brushes as it enables the brush to quickly work to proper position to conform to the commutator. v

Other and further objects and advantages of the invention will .be hereinafter set forth and the novel features thereof defined by the appended claims.

In the drawing:

Figure 1 is a longitudinal section through a motor showing the improved construction of brush assembly applied thereto;

Figure 2 is an enlarged section greatly exaggerated showing the cocking of the brush within the holder; and

Figure 3 is a perspective view showing the angles of beveling the end of the brush in two directions.

Referring to the drawing I indicates a motor casing herein shown formed of two sections to allow the same to be assembled or taken apart. The ends of the sections are provided with selfcentering bearings 2 in which is mounted a shaft 3 carrying an armature 4 surrounded by field structure 5. The shaft 3 carries a commutator 6 engaged by brushes as will be later described, the above description being given so that the operation and advantages obtained by my improved construction of brush will be readily understood.

Fixed within the casing above and below the commutator are tubular brush holders 7 constructed identically alike and the description of one will be sufficient for both.

The tubular brush holder as herein shown is rectangular in cross section and is provided with a slot 8 through which extends a lead wire 9 fixed to the upper end of a rectangular brush l0 having its lower end beveled in two directions to form a point contact with the commutator against which it is held under spring pressure by a coil spring ll held within the brush holder by bending a portion of the wall thereof inwardly as shown at Hi.

In order to produce an operative structure, it is necessary that the brush be formed with a cross section area slightly less than the cross section area of the inside of the holder so that it can move up and down freely therein and as. the clearance varies caused by the inaccuracy in manufacturing the brush and holder, it is necessary to provide means for preventing the brush from vibrating within the holder and causing a noise and prior to my invention, devices such as springs and rubber cushions of various constructions have been tried without success.

In constructing a brush in accordance with the invention herein shown, the contacting end of the brush is beveled in two directions which are preferably struck on the arc of a circle, one bevel being approximately 6 and. the other bevel being approximately 9 as shown in Figure 3 of the drawing, which not only reduces the area of contact between the brush and commutator but causes the brush to become cooked in the holder and ride on one side of the holder due to the shape of the end and the pressure of the spring, thereby causing a wedging action of the brush within the holder which prevents the brush from vibrating in the holder and causing a rattling or chattering noise and yet allows the brush a free longitudinal movement within the holder to make the proper contact between the brush and commutator.

While I have obtained the best results with a brush beveled at the angles substantially as specified, it is, of course, understood that I do not wish to limit myself to the specific angles herein specified as these angles can be varied slightly without departing from the spirit of my invention.

It has proven experimentally that by beveling the end of the brush in two directions that in the mass production of motors the inaccuracies in the construction of the brush and holder are compensated for, thereby eliminating any machining operations whereby a motor can be manufactured very cheaply and yet be quiet in operation. I

In the operation of a motor constructed in accordance with my invention, when the brush is placed in position Within the holders and is forced into engagement with the commutator, the beveled end causes the brush to cock within the holder to cause a wedging action and as a point contact with a small area is produced, this point soon wears to conform to the irregularities of the commutator and as the brush is free to move up and down and yet be wedged within the brush holder, brush noise is eliminated.

From the foregoing description it will be seen that I have provided a construction of brush assembly in which the brush noise will be eliminated by simply beveling the end of the brush in two directions to cause the brush to cook within the holder.

What I claim is:

l. The combination of a dynamoelectric machine having a commutator, at least one rectangular brush for contacting the commutator, a tubular brush holder whose cross section corresponds to, but is substantially larger than, the cross sectional configuration of the brush, and resilient means for holding the brush in contact with the commutator, the end of the brush contacting with the commutator being cut at a bevel on each of two adjacent sides, the radius of curvature of each bevel forming an acute angle with the longitudinal axis of the brush.

2. The combination of a dynamoelectric machine having a commutator, at least one rectangular brush for contacting the commutator, a tubular brush holder whose cross section corresponds to, but is substantially larger than, the cross sectional configuration of the brush, and resilient means for holding the brush in contact with the commutator, the end of the brush contacting with the commutator being cut off at an acute angle and beveled on each of its two adjacent sides from one corner thereof, one bevel being at an angle of substantially 6, the other bevel being at an angle of substantially 9 to form a substantially triangular shape contact between the commutator and brush.

3. The combination of a dynamoelectric machine having a commutator, at least one rectangular brush for contacting the commutator, a tubular brush holder whose cross section corresponds to, but is substantially larger than, the cross sectional configuration of the brush, and resilient means for holding the brush in contact with the commutator, the end of the brush contacting with the commutator being cut off at an acute angle to form a substantially fiat face and beveled on each of two adjacent sides to form a substantially triangular shaped contact between the commutator and brush for cooking said brush within the holder.

ALBERT G. REDMOND. 

